Any comments on this design?

icebear · 12th October 2004, 12:36 PM

Any opinions on this circuit. It's from diyparadise. I would go with triodeconnection on all the tubes. That will give like 5 wattt. Do anyone have experince with EL84 in SE vs PP ?

Bjorn

SY · 12th October 2004, 12:49 PM

By making the output stage double as the phase splitter, you lose some of the advantages of push-pull, most notably cancellation of second (and other even order) harmonic distortion. At the cost of efficiency, complication, and reliability, the CS will help mitigate this.

The logic behind this is questionable.

SY · 12th October 2004, 03:27 PM

On further review, I'm wrong about distortion. The CS does mitigate this. But I'm still right on inefficiency, heat, complication, and reliability.

Sch3mat1c · 12th October 2004, 06:30 PM

Well, it's always going to be as inefficient and unreliable because you've got class A there.

It'll work fine as long as gain is enough... it should take twice the signal drive as normal PP, I think.

Tim

resident · 13th October 2004, 06:56 AM

A nice two stages P-P amp!
Go with an UL tranny, so you can hear it triode ,pentode and UL!Also try it with G2 ( screen ) something lower than 300V, say 200-250V.Just for an experiment.Some people like it better than pentode or UL.
Also try it without 470u.I thing this cap doesn't belong there.Read at diyparadise the original article about this amp.There's a reference about this cap that I don't remember.

Klimon · 16th May 2009, 07:17 PM

Quote:

it should take twice the signal drive as normal PP

Is this correct?

Thanks!

Simon

revintage · 16th May 2009, 07:32 PM

It is correct, you loose 6dB. But his how all LTP, long tail phase inverters work.

But I would do a "Schade" on the output EL84s and choose another driver like 6C45 or 6H30.

JohanB · 16th May 2009, 08:32 PM

Electronic World published this circuit in 1961 with a 6C4 as input driver tube. This was before the time of low voltage ccs's, so there is just a resistor in the cathodes.
You can actually see this as a single ended amplifier (V2) with a opt current equalizing tube (V3) and active load. Ofcourse the amplitude on V3 anode will be lower than on V2, but the circuit vill cancel a lot of distorsion anyway. The second harmonic of the 6C4 is also in opposite phase to that in V2.
I think this circuit could also be tried with 6CW5/EL86's as output tubes, in triode or pentode operation, with even better performance than the 6BQ5/EL84. For a budget project, the low priced NOS 15CW5/PL84 are plentiful. same specs as 6CW5/EL86 despite the number PL84!! The only difference is that it takes 15V at 300mA for the heater.

http://triodeel.com/compact.html

JohanB

JohanB · 16th May 2009, 10:41 PM

I sew that this first circuit in diyparadise with CCs was revised by someone later on, back to a common resistor in the cathodes (as in the original Leibowitz circuit). See link below.

http://diyparadise.com/web/index.php...d=58&Itemid=26

I'm still puzzled about using power tubes (6BQ5 or later 1/2 5687) for driving the grid of the output tube. I think it's an overkill (and expensive) to use as driver, especially in pentode and UL-mode.

If not using a 6C4 (EC90) in the pentode mode operation, 1/2 12AT7/ECC81 (EC92/6AB4) will have enough gain and power also for the millercaps in triode mode. Will also give a nice 2:nd harmonic to partly cancel the resulting 2:nd harmonic in the first output tube.

JohanB

Poindexter · 17th May 2009, 02:42 AM

I have two thoughts on this setup:

Yes, singly driving a diff-amp with a long tail causes the cathode of the driven tube to follow the grid halfway, so we're dividing the gain of the driver tube between the two outputs tubes.  An EL84 in the 'first hole', as shown is not gonna get you there without a truly monster signal coming in the front - much greater than line level.  A bigger gain low rp tube like the 6GK5 or similar would ameliorate this problem.

The CCS in the output tail does solve the horrendous output signal imbalance problem of the original Compact, but I don't know how thick on the ground CCS circuits are that can stay firmly in regulation at 80mA with only the 18v (or so) cathode lift of the EL84 outputs across them.  Maybe somebody else here, like Stuart, can comment on this.  If it don't fly, we could always build a little 50v or so negative supply for the tail.

Otherwise, why not try it? All the parts could be recycled if it doesn't sound good.

Aloha,

Poinz
AudioTropic

12th October 2004, 12:49 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	By making the output stage double as the phase splitter, you lose some of the advantages of push-pull, most notably cancellation of second (and other even order) harmonic distortion. At the cost of efficiency, complication, and reliability, the CS will help mitigate this. The logic behind this is questionable. __________________ If there's a sucker born every minute, where do the rest of them come from?

12th October 2004, 03:27 PM	#3
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	On further review, I'm wrong about distortion. The CS does mitigate this. But I'm still right on inefficiency, heat, complication, and reliability. __________________ If there's a sucker born every minute, where do the rest of them come from?

12th October 2004, 06:30 PM	#4
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	Well, it's always going to be as inefficient and unreliable because you've got class A there. It'll work fine as long as gain is enough... it should take twice the signal drive as normal PP, I think. Tim __________________ Seven Transistor Labs Projects and Resources

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13th October 2004, 06:56 AM	#5
resident Account Disabled Join Date: Sep 2004 Location: Earth	A nice two stages P-P amp! Go with an UL tranny, so you can hear it triode ,pentode and UL!Also try it with G2 ( screen ) something lower than 300V, say 200-250V.Just for an experiment.Some people like it better than pentode or UL. Also try it without 470u.I thing this cap doesn't belong there.Read at diyparadise the original article about this amp.There's a reference about this cap that I don't remember.

16th May 2009, 07:32 PM	#7
revintage diyAudio Member Join Date: May 2007	It is correct, you loose 6dB. But his how all LTP, long tail phase inverters work. But I would do a "Schade" on the output EL84s and choose another driver like 6C45 or 6H30.

16th May 2009, 08:32 PM	#8
JohanB diyAudio Member Join Date: May 2009 Location: Stockholm	Electronic World published this circuit in 1961 with a 6C4 as input driver tube. This was before the time of low voltage ccs's, so there is just a resistor in the cathodes. You can actually see this as a single ended amplifier (V2) with a opt current equalizing tube (V3) and active load. Ofcourse the amplitude on V3 anode will be lower than on V2, but the circuit vill cancel a lot of distorsion anyway. The second harmonic of the 6C4 is also in opposite phase to that in V2. I think this circuit could also be tried with 6CW5/EL86's as output tubes, in triode or pentode operation, with even better performance than the 6BQ5/EL84. For a budget project, the low priced NOS 15CW5/PL84 are plentiful. same specs as 6CW5/EL86 despite the number PL84!! The only difference is that it takes 15V at 300mA for the heater. http://triodeel.com/compact.html JohanB

16th May 2009, 10:41 PM	#9
JohanB diyAudio Member Join Date: May 2009 Location: Stockholm	I sew that this first circuit in diyparadise with CCs was revised by someone later on, back to a common resistor in the cathodes (as in the original Leibowitz circuit). See link below. http://diyparadise.com/web/index.php...d=58&Itemid=26 I'm still puzzled about using power tubes (6BQ5 or later 1/2 5687) for driving the grid of the output tube. I think it's an overkill (and expensive) to use as driver, especially in pentode and UL-mode. If not using a 6C4 (EC90) in the pentode mode operation, 1/2 12AT7/ECC81 (EC92/6AB4) will have enough gain and power also for the millercaps in triode mode. Will also give a nice 2:nd harmonic to partly cancel the resulting 2:nd harmonic in the first output tube. JohanB

17th May 2009, 02:42 AM	#10
Poindexter diyAudio Member Join Date: Oct 2004 Location: Maui, Hawai'i, USA	I have two thoughts on this setup: Yes, singly driving a diff-amp with a long tail causes the cathode of the driven tube to follow the grid halfway, so we're dividing the gain of the driver tube between the two outputs tubes.  An EL84 in the 'first hole', as shown is not gonna get you there without a truly monster signal coming in the front - much greater than line level.  A bigger gain low rp tube like the 6GK5 or similar would ameliorate this problem. The CCS in the output tail does solve the horrendous output signal imbalance problem of the original Compact, but I don't know how thick on the ground CCS circuits are that can stay firmly in regulation at 80mA with only the 18v (or so) cathode lift of the EL84 outputs across them.  Maybe somebody else here, like Stuart, can comment on this.  If it don't fly, we could always build a little 50v or so negative supply for the tail. Otherwise, why not try it? All the parts could be recycled if it doesn't sound good. Aloha, Poinz AudioTropic

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